The rownum has one side effect. You cannot select a row number that is greater than a particular number. For example, if you want all but the first two rows and you execute the following statement:
SELECT ROWNUM, Empno, ename
WHERE ROWNUM > 2;
you receive the following message:
no rows selected
The reason is that when the first row is returned, it is given the ROWNUM value of 1. It is tested against the WHERE clause and then rejected. The next row is then given the ROWNUM value of 1. It is tested and also rejected. This continues until no more rows are in the table, and no rows are returned in the result set.